The present invention is directed toward solving the problem of stress related to the packaging configuration for a diaphragm-type absolute pressure sensor which includes a vacuum reference.
The prior art is descriptive of a silicon based absolute sensor structure which employs implantable semiconductor strain sensitive resistors (utilizing the commonly referred to piezoresistive effect), a stress isolating structure, for example, a Pyrex tube to which the silicon pressure die is anodically bonded, and a vacuum reference, which is made during encapsulation of the sensor within the sensor package. The vacuum reference is maintained over time by the use of a discrete getter attached to the cover of the sensor package and "activated" during the encapsulation process. The package provides the mechanical pressure port interconnect between the strain sensor and the outside world and the electrical interconnect between the silicon piezoresistors (strain resistors) on the silicon die and the outside world.
The strain sensitive piezoresistors are arranged on the silicon die and typically interconnected so as to form a full Wheatstone bridge on one surface. The underside of the silicon die is usually etched to form a thin diaphragm, in this case circular. The edges of the diaphragm correspond to the location of the piezoresistors, where strain is the greatest when the diaphragm undergoes bending as a result of pressure loading on one side. The desired sensitive axis is perpendicular to the diaphragm.
Since the sensor is also sensitive to package induced strain effects, a mechanical strain isolator must be employed to maintain pressure sensitivity selectivity. A Pyrex tube or washer, or a secondary and/or tertiary silicon strain isolation chip is often employed between the sensor and package.
The absolute pressure sensor accuracy over time and temperature is a function of the quality of the vacuum. The lower and more stable the vacuum, the better the accuracy and repeatability of the transducer over time. The vacuum is established during package cover seal at which time a getter is installed and activated. The cover is usually sealed by laser welding in a vacuum.
The attachment of the sensor die (chip) to the Pyrex tube, insertion of the chip/tube into the package, and vacuum processing of the package is done in discrete steps, which are labor intensive and therefore expensive.
Thus, a need exists for a reduced-cost apparatus for isolating a diaphragm-type absolute pressure sensor from the stress introduced by the sensor package.